An electric vehicle, including a hybrid vehicle, which has an alternating current electric rotary machine for a vehicle power source is known. The patent document U.S. Pat. No. 6,917,179 (JP2003-244801A) discloses an apparatus which includes an alternating current electric rotary machine for driving a driving wheel of the vehicle, and an alternating current electric rotary machine for being driven by an internal combustion engine for generating electricity. The patent document U.S. Pat. No. 7,099,756 (JP2004-274945A) discloses a similar apparatus. The alternating current electric rotary machine may be referred to as the AC machine. This kind of vehicles carries a rechargeable battery as a direct current power supply. The direct current power supply may be referred to as the DC power supply. The apparatus includes a converter which increases a voltage of the DC power supply and supplies the increased voltage to a power line. The AC machines are connected to the power line via inverters. The increased voltage is converted into an AC voltage by the inverter and supplied to the corresponding AC machine. On the other hand, an AC voltage generated on the AC machine is converted into a DC voltage by the inverter and supplied to the power line. The converter may charge the battery by decreasing the voltage on the power line. Therefore, the converter and the inverter are used for both a supplying purpose and a regenerating purpose. The voltage generated on the power line is also referred to as a system voltage.
The patent document U.S. Pat. No. 7,355,869 (JP2005-51898A) discloses an apparatus which has a voltage regulator for stabilizing the system voltage. The voltage regulator monitors the system voltage by detecting the system voltage, and controls the converter in a feed-back control fashion. The converter is controlled so that a difference between a detected voltage and a target voltage becomes small. In other word, the converter is controlled so that the detected voltage approaches to the target voltage.